What you need to know about a storm that brings high winds, heavy rain, snow to California and the Pacific Northwest

SAN FRANCISCO (AP) — One of the strongest storms on the West Coast for decades knocked out power to thousands of people, unleashed strong winds that toppled trees and left two dead in Washington before making its way through Oregon to northern California, where heavy snow and record rain fell on Thursday.

The National Weather Service extended a flash flood watch through Saturday for areas north of San Francisco as the region was inundated by the strongest atmospheric river — a long and wide plume of water that forms over an ocean and flows over land — so far this season.

The storm system roared ashore Tuesday as one “bomb cyclone,” which occurs when a cyclone intensifies rapidly, and is expected to bring moderate to heavy rain through Saturday, increasing the risk of flash flooding and rockslides, forecasters said.

Here are some things to know about the storm:

Strong winds cause power outages and knock down trees

About 285,000 homes and businesses remain without power Thursday in Washington, where falling trees hit homes and littered roads across the western part of the state, killing at least two people. A woman in Lynnwood died when a large tree fell on a homeless camp, while another woman in Bellevue was killed when a tree fell on a home.

Cities began opening warming centers that offer free internet and device charging. In the hardest-hit areas east of Seattle, power was not expected to be fully restored until noon Saturday, Puget Sound Energy said.

At least a dozen schools were closed in the Seattle area Wednesday, and some chose to extend those closures into Thursday. In Enumclaw, east of Seattle, residents were cleaning up after their town had the highest winds in the state Tuesday night: 74 mph (119 km/h).

Meanwhile, in Northern California, there were reports of power outages Thursday affecting more than 20,000 customers. Several districts in Sonoma County closed their schools Thursday because of the storm, county officials said.

Officials warn of dangerous travel conditions

About 150 flights were delayed and another two dozen canceled Thursday at San Francisco International Airport, after hundreds were delayed and dozens canceled Wednesday, according to flight tracking service FlightAware.

Transportation officials closed a 2-mile (3.2 kilometer) stretch of Northern California’s famed Avenue of the Giants, a scenic drive named for the towering redwoods along the route, due to flooding. The National Weather Service warned that road conditions would be hazardous in the Sierra Nevada.

“The hazardous conditions may affect commutes Friday night and Monday morning as roads become slippery and snowy,” the service said. “Gusty winds could bring down tree branches.”

A winter storm watch was in place for the northern Sierra Nevada above 3,500 feet (1,100 meters), where 15 inches (38 cm) of snow was possible over two days. Gusts could top 75 mph (120 km/h) in mountainous areas, forecasters said.

Maggie Eshbaugh, marketing manager at Sugar Bowl Resort, northwest of Lake Tahoe, said a foot (30 centimeters) of snow fell Wednesday night. She said they are excited to welcome customers Friday, which is the earliest the ski resort has opened in 20 years.

Interstate 5 was closed Wednesday morning for an 11-mile (18-kilometer) stretch from Ashland, Oregon, to the California border due to extreme winter weather in Northern California, according to the Oregon Department of Transportation. It was reopened on Wednesday evening.

The storm also caused some damage on State Highway 6 near the Oregon coast, but it appeared to largely spare the state, with no major damage reported as of Thursday.

Rain and snow may continue next week

A second, lighter wave of the storm is expected from Saturday into Tuesday in Northern California, said meteorologist Courtney Carpenter of the National Weather Service Sacramento. It will bring lighter rain in the lower elevations, but heavier mountain snow that will reach lower levels than the first wave, she said.

Dangerous flooding, rockslides and debris flows were possible, especially where hillsides were loosened by recent wildfires, officials warned. Scott Rowe, a hydrologist with the weather service in Sacramento, said so far the soil has been able to absorb the rain in California’s Butte and Tehama counties, where the Park Fire burned over the summer.

“It’s not necessarily how much rain falls; it’s how fast the rain comes down,” Rowe said Thursday.